Technical Field
The present invention relates to structures of display pixels in horizontal electric field type liquid crystal display devices (LCDs), as represented by fringe field switching mode (FFS) and in-plane switching mode (IPS).
Related Art
In horizontal electric field type LCDs, light transmittance is controlled by applying an electric field between pixel electrodes to rotate liquid crystal molecules from an initial alignment direction.
In the above LCDs, an alignment angle of the liquid crystal molecules changes depending on which direction the liquid crystal molecules rotate: right or left, whereby optical characteristics also change.
In addition, when a rightward rotating liquid crystal molecule region and a leftward rotating liquid crystal molecule region are formed in a single region, a specific region, called “disclination”, occurs at a boundary between the regions. The disclination does not contribute to light transmission and thus degrades transmittance, or depending on a shape of the occurred specific region, variation occurs in transmittance of each pixel, thereby causing a problem of display unevenness.
Accordingly, in conventional horizontal electric field type LCDs, in order to specify a rotation direction of liquid crystal molecules at a time of application of voltage with respect to an initial alignment direction (a rubbing direction or a photo alignment direction) of the liquid crystal molecules, an extension direction of pixel electrodes and counter electrodes has been oriented obliquely with respect to the initial alignment direction so that the direction of electric field is not perpendicular to the initial alignment direction.
Japanese Patent No. 3267224 (Patent Literature 1) is an example of the related art, and Japanese Unexamined Patent Application Publication No. H7-43721 (Patent Literature 2) is another example of the related art.
FIGS. 7 and 8 depict examples of pixel structures of conventional ordinary horizontal electric field type liquid crystal elements; FIG. 9 is an illustrative view depicting a disorder of a liquid crystal initial alignment state at a pixel electrode side wall portion; FIG. 10 is an illustrative view depicting a polarization state of reflected light when linearly polarized light is input to and reflected by a wiring metal side wall portion; and FIG. 11 is an illustrative view depicting a direction of an electric field and a rotation direction of a liquid crystal molecule in a vicinity of an end part of a pixel electrode.
Due to the restriction mentioned above, as depicted in the examples of FIGS. 7 and 8, the direction of the electrodes is oblique with respect to the initial alignment direction, so that an entire shape of the pixels also becomes a shape like a combination of shapes having an oblique inclination with respect to the initial alignment direction. Specifically, it is inevitable to form a complicated shape, such as “a dogleg shape” (FIG. 7) or “a combination of a plurality of dogleg shapes” (FIG. 8), which has been a factor causing reduction in aperture ratio. In FIG. 7, the pixels are formed into a distorted shape due to the oblique electrode shapes. On the other hand, in FIG. 8, the pixels have a rectangular shape in which the presence of a dead space significantly reduces the aperture ratio.
In addition, as depicted in FIG. 9, the initial alignment direction and an extension direction of a stepped part around a pixel electrode are not parallel or perpendicular to each other. Thus, in an extreme vicinity of the electrode stepped part, an influence of a shape of the stepped part causes the initial alignment direction to be oriented in an extension direction of the pixel electrode, thereby causing leakage of light. Reference sign 11g denotes a liquid crystal molecule whose alignment direction has shifted due to the influence of the stepped part.
In addition, as depicted in FIG. 10, when a pixel electrode, a counter electrode, a signal line, and the like are made of metal and, at a time of reflection of linearly polarized light impinging to a side wall of the electrode or the wire, a polarization direction and a direction of the side wall are not parallel or orthogonal to each other, a polarization angle of the reflected light changes, which has been a factor causing leakage of light. An arrow of reference sign 13a denotes a polarization state of the incident light, and an arrow of reference sign 14a denotes a polarization state of the reflected light.
In addition, in the conventional horizontal electric field type LCDs, as described above, the rotation direction of liquid crystal molecules are specified by arranging the pixel electrodes and the counter electrodes obliquely with respect to the initial alignment direction to generate the direction of the electric field obliquely. However, as depicted in FIG. 11, in the vicinity of the end part of the electrode, the generation direction of the electric field radically changes to turn to the opposite rotation direction, thereby generating a region in which a part of the liquid crystal molecules are oppositely rotated. Reference sign 11h denotes an oppositely rotating liquid crystal molecule, and an arrow of reference sign 11i denotes a direction in which the electric field is easily rotated.
Additionally, since an angle is formed in the direction of the electric field with respect to the initial alignment direction of the liquid crystal molecules, rotation torque efficiency is degraded. Thus, it has been necessary to apply high voltage.
As means for solving the problems mentioned above, Patent Literature 1 in the conventional art discloses a technique in which slits are formed on insulating films provided between a pixel electrode and a common electrode, an extension direction of each slit is arranged in a direction opposite to a desired liquid crystal rotation direction to thereby bend the direction of an electric field passing through the slit in a direction perpendicular to the extension direction of the slit. This technique can provide an electric effect substantially equivalent to that obtained by arranging the electrodes obliquely with respect to an initial alignment direction. Thus, there can be obtained an advantageous effect in that the initial alignment direction and the extension direction of the pixel electrode can be made substantially parallel to each other.
In the above technique, however, the extension direction of the slit is arranged in the direction opposite to the desired liquid crystal rotation direction. Due to this, liquid crystal molecules near an edge of the slit aligned along the edge thereof tend to be aligned in a rotation direction opposite to an originally desired liquid crystal rotation direction. Thus, there has been a problem of increased risk of causing opposite rotation.
Additionally, regarding a structure that changes an alignment state by forming a plurality of projections on a pixel electrode substrate or a counter substrate, there is a known structure as disclosed in Patent Literature 2. However, the technique of Patent Literature 2 has been proposed to make large a mean value of pre-tilt angles of liquid crystal on the substrate, and thus does not consider a planar shape and a planar anisotropy of the projection structure.
Furthermore, in the above arrangement, the projection structure does not have any planar anisotropy, as mentioned above, and therefore has no influence on a rotation direction of liquid crystal molecules in the horizontal electric field type liquid crystal display device.